soil types, footing weight and code
Hi all:
I’m miserable. I’m trying to design a barn on the cheap, and I’m running into some roadblocks. To minimize expense I’m trying to use a pier foundation with sonotube and either Bigfoot or another similar footing form like it. I need to know how much weight I can put on the various sizes of the forms (20″, 24″, 28″, 36″) in order to hold up my barn.
My building official (small town) has the personality of a brick wall and won’t give me the time of day or offer an opinion before the plans are drawn up, although he certainly had a specific opinion on the footing size for a recent deck I did for a local client. He specified 2300 pounds for a 12″ sonotube in that instance.
I’ve researched my soil type– it is Penn Bucks Complex, which is decribed as “silt loam.” It has some clay, some silt, I guess. Terrific, I says to myself, now let me look up what kind of weight my soil will support. That’s where my first roadblock comes up. The 2003 IRC (in NJ we use the 2000 IRC, but I could only find the 2003 version to purchase) classifies five different levels of soil (1804.2) but nowhere does it mention “silt loam.” Does anyone know what kind of loads I can put on silt loam or where it falls in the IRC chart?
I guess my fallback is to just do a slab foundation, but the estimate I got for that was $11k and I was really hoping to not have a concrete floor. This barn is for a woodworking shop and wood floors are more forgiving. I’d hire an architect and be over with it, but I don’t have the money for that either. Any help or pointers is appreciated…
John Painter
Replies
I just hate misery. Especially when it happens to enduring charmers.
Maybe we can all help you banish it.
Structural advice is normally shunned here because of discalimers for liability and it seems nobody ever comes through with the needed information for complete calculations so answers cantend to be minimal.
To be fair, it is not the inspectors job to spec or to deign for you.
now on the soils - and disclaimer #1, I am far from a soils engineer - is it also the silt and loam mix down four feet where the beaaring point will be? Loam is generally at the surface, another name for topsoil a mix of humus and other. Whoever advises on the load bering capacity will need to know specificly what it is at the bering depth on site, not what a geo study map indicates is prevailing.
Maybe that is what is there but...
Not challenging you, just informing and getting the ball rolling.
Then we would need to know the size of the building, the required live lods and ded loads for this building and the projected number of piers. All these vriaables interct with the foot size to formulate the answers you need.
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P:
Yeah, I realize all that. The inspector doesn't owe me anything, but the good ones will meet you half way and at least talk to you like a human being. This one was perfectly willing to inform me what a 12" footing (in his opinion) will support in our area, but a week later he wouldn't have an informal conversation about footings with me or point me to another source. In a small town like mine where I pay municipal taxes into the five figures annually, I'd like to think it isn't too much to ask for some common courtesy.
Anyway, the rest of the barn is really not the issue. In other words, I have a handle on the framing and on how much I'll have as far as live/dead loads, tributary loads, etc. The question is: how much weight can I put on one of these preformed footings in the sizes they come in? I can move them closer together or go up a size to accomodate the building loads, but I just don't know what they can support. (The barn is 24x36 with a shed addition off the back if you are curious.) This "Penn Bucks complex" soil shoud have an allowable foundation pressure expressed in PSF associated with it just like any other soil, but I just don't know where to find the number.
The Bigfoot website has an old chart referencing the BOCA code, but, again, that doesn't help me figure out where my soil fits in. Incidentally, I found a website that will identify soil type right down to a single residential address:
http://websoilsurvey.nrcs.usda.gov
Pretty cool, but not all the way there for my purposes. At this point I'm leaning toward just going with a slab foundation. I think the inspector will have less of an issue with that, and it will make my framing easier to design and build. It's just a shame because I really don't want all the disruption that a slab will do to my yard (gotta put all that dirt somewhere) and it is probably three times the cost. Plus, a concrete floor isn't what I had in mind.
My wife mentioned moving out of NJ when I told her of this dilemma and for the first time I hesitated before answering. I'd probably be happier in northern New England somewhere, but with two kids in school I don't think its in the cards right now. Oh me, oh my..
John
I may well be wrong but I believe that a minimum soils bearing was considered to be 2500 psf. in the older codes here. Might try basing a design on that and see if the BI corrects it. Or try calling a local geo tech firm and asking for some input ."Poor is not the person who has too little, but the person who craves more."...Seneca
Edited 7/5/2007 8:02 pm by dovetail97128
D:
That must have changed. The 2003 IRC has five classes of soils, and the last two are 2000 PSF and 1500 PSF, respectively. I have a feeling that my soil is in the fourth catagory (2000 PSF) but I just can't confirm it. Too bad I don't have an engineer who owes me a favor.
J.
<<Too bad I don't have an engineer who owes me a favor.>>If your alternative to hiring an engineer is to pour a slab why not hire the engineer? even at $200 per hour it's cheaper than the slab. I'm in NC pouring on expansive clay soils at 12" frost depth and we build pole barns all the time on 24"x24"x10" footings under 6x6 poles generally 7' to 10' O.C. I do get my pole barns engineered as needed for the beams but have never had a problem with the pier support. I'd suggest you draw up your plans, print them out, write a list of questions you want answered by your engineer to define his scope of work. and have him write you a letter stating that he has reviewed the plans and in his opinion the beams need to be such and such size and the footings need to be such and such size. Have him stamp the letter and attach it to your drawings. This relieves him of responsibility for other items in the plans that might not be addressed by his letter but would be covered by code. Thus you are not asking him to "stamp the drawings" only to "give an opinion on certain elements of interest to the building inspector" this saves him time, limits his liability and saves you money. All the bestMichael------------------
"You cannot work hard enough to make up for a sloppy estimate."
It seems like hiring a soils engineer would be cheaper thaan moving out of town...
these might have some value for you.
http://www.sbe.napier.ac.uk/esm/chapter8/index.html
https://www.asce.org/bookstore/book.cfm?book=2548
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
(71) Bucks silt loam
This silty and clayey soil occurs on hilltops in weathered shale and sandstone. Hard bedrock is three to eight feet below the surface. Permeability is moderate, but may be restricted by un-fractured bedrock in some areas. Foundation support is generally good. Grading and subsurface drainage may be needed to prevent wet yards. Septic drainfields and infiltration trenches may be limited in areas of shallow bedrock. Use of this bedrock in engineered fill, road embankment, or trench backfill is limited due to rapid disintegration.
(73) Penn silt loam
This silty soil occurs on hilltops and sideslopes over red sandstones and shales. Depth to hard bedrock is less than three feet. Permeability is moderate to moderately rapid, but may be restricted by un-fractured bedrock. Foundation support is good. Grading and drainage may be needed to prevent wet yards. Shallow bedrock may limit septic drainfields and infiltration trenches. The bedrock disintegrates rapidly, limiting its use in engineered fill, road embankment, or trench backfill. Topsoil may be needed to increase rooting depths for lawns, trees, and landscape plants.
From:
http://www.fairfaxcounty.gov/dpwes/environmental/soil_d.htm
So if you make a test bore you might find yourself on bedrock before reaching the frost depth requirement for your area. Then you don't need to be concerned with what is known as Prime Farmlands soil and its bearing capacity.
Drill the bedrock for pins to help anchor your footing.
that is what I was trying to get at.My property has two different soils areas for surface soil, but when you dig down more than two feet it is almost all the same blue clay except for over htere where it is more of a red clay...LOLAnyway, I browsed away from the page you linked to this one
http://www.fairfaxcounty.gov/dpwes/environmental/soilrating.htm#good
and found those soils of his listed as GOOD for foundation support. All my Googling points to onsite aanalysis and testing of soils to determine the load bearing capacity and no charts to give a certain number, which is what I expected
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P:
You and Ralph make good points. Maybe I'll ask around to see if anyone knows an engineer who will work on an hourly basis. You know, it is so difficult to stay focused on your own projects when you spend all day working on other people's homes. You lose perspective. Plus, this building official's attitude has me burned up. I could go to the towns on either side of mine and the building officials there would be happy to work with me to figure out an acceptable solution.
Failing to find where my soil fits into the IRC chart I guess I'll try to find an engineer or else consider going with a slab or even hiring an architect for my lousy barn. Hey, its only money, right? I'll just make more!
John Painter
enduring ,
If you get along with the neighboring BI's what I would do is ask them.
I often do that when faced with a tricky situation that I want input on without tipping my hand about it to whichever BI here I am working under ."Poor is not the person who has too little, but the person who craves more."...Seneca
Not meaning to sidetrack you, but if you work up the complete structure for your barn, you may find your problem disappears.
Selecting a bay size that gives you small economical and light beams, with more frequent posts may mean each footing isn't picking up much. It also has the added bonus that you don't have to buy a forklift from Frenchy to erect it.
We are allowed to assume 1500 PSF here for soil bearing as long as the organics are removed and the soil beneath is undisturbed. How much is your building going to weigh, and how many piers were you thinking?
David:
Yeah, I think 1500 PSF is sort of a minimum unless it is obvious there is just loose organic material for soil. The barn I was designing is not insignificant, being 24x36 with a 12 foot shed addition off the back and an extra 8 feet on one side due to a saltbox arrangement. Plus, it is one and a half stories and a wood floor on the first story. One version I had would mean that each pier would take around 9-10K pounds, which is in line with a 24 or 28 inch footing, depending on what the soil can bear. If it is only 1500 PSF then it won't work. At that minimum I'd be digging more holes than it is worth.
John Painter